For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Quenching of oxygen(1.DELTA.8) by amines

Physical mechanisms of generation and deactivation of singlet oxygen.

CHAPTER 3 – Photosensitized Oxidation and Singlet Oxygen: Consequences in Biological Systems

SINGLET oxygen is receiving increasing attention as a reactive species in many chemical or biological reactions (see refs 1–3 for recent reviews). We report here a new method for detecting the formation of singlet oxygen by the generation of stable nitroxide radicals from sterically hindered amines.

New method of detecting singlet oxygen production

Advances in singlet oxygen chemistry

Singlet molecular oxygen, O2(a1Δg), has been detected from single neurons and HeLa cells in time-resolved optical experiments by its 1270 nm phosphorescence (a1Δg → X3Σ–g) upon irradiation of a photosensitizer incorporated into the cell. The cells were maintained in a buffered medium and their viability was assessed by live/dead assays. To facilitate the detection of singlet oxygen, intracellularH2O was replaced with D2O by an osmotic de- and rehydration process. The effect of this insult on the cells was likewise assessed. The data indicate that, in the complicated transition from a “live” to “dead” cell, the majority of our cells have the metabolic activity and morphology characteristic of a live cell. Quenching experiments demonstrate that the singlet oxygen lifetime in our cells is principally determined by interactions with intracellularwater and not by interactions with other cell constituents. The data indicate that in a viable, metabolically-functioning, and H2O-containing cell, the lifetime of singlet oxygen is ∼3 µs. This is consistent with our previous reports, and confirms that the singlet oxygen lifetime in a cell is much longer than hitherto believed. This implies that, in a cell, singlet oxygen is best characterized as a selective rather than reactive intermediate. This is important when considering roles played by singlet oxygen as a signaling agent and as a component in events that result in cell death. The data reported herein also demonstrate that spatially-resolved optical probes can be used to monitor selected events in the light-induced, singlet-oxygen-mediated death of a single cell.

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Quenching of oxygen(1.DELTA.8) by amines

Citations

Physical mechanisms of generation and deactivation of singlet oxygen.

CHAPTER 3 – Photosensitized Oxidation and Singlet Oxygen: Consequences in Biological Systems

New method of detecting singlet oxygen production

Advances in singlet oxygen chemistry

Measuring the lifetime of singlet oxygen in a single cell: addressing the issue of cell viability.

Related Papers (5)

Quenching of singlet oxygen by tertiary aliphatic amines. Effect of DABCO (1,4-diazabicyclo[2.2.2]octane)

Rate Constants for the Decay and Reactions of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution. An Expanded and Revised Compilation

Kinetics of Fluorescence Quenching by Electron and H‐Atom Transfer

Photosensitized oxygenations and the role of singlet oxygen

Physical and chemical properties of singlet molecular oxygen